NATO forces carry a corpse from the site of a suicide attack near Darul Aman Palace in Kabul on October 29, 2011.

U.S. forces in Afghanistan have suffered their "deadliest insurgent attack in months," the Guardian reports, with 13 Americans killed in Kabul after a car filled with explosives rammed into the side of an armored bus transporting troops.

The International Security Assistance Force (ISAF) confirmed that five soldiers and eight civilian contractors were killed in the attack on the Rhino bus traveling between NATO bases.

However, Afghan and western officials privately confirmed that all of the dead were from the U.S., making it the heaviest loss of American lives since a Chinook helicopter was shot down by the Taliban in August, killing 30 Americans and eight Afghans.

The size of the explosion reportedly made identification of victims difficult.